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Monday, December 24, 2012

Beat Your Meter

Override your camera’s TTL light meter when atypical scenes demand it

This Article Features Photo Zoom

Digital Photo Tip Of The Week
Understanding how your digital camera's through-the-lens light meter works will help you make more accurate exposures, no mater what metering mode or exposure mode you're using. Unlike handheld meters that measure incident light (the illumination falling on a subject) TTL camera meters measure reflected light—the light that bounces off a subject and into the camera lens. That's all well and good, except that camera meters can be fooled simply because they assume light is always bouncing off of a medium gray surface of 18% reflectivity. Most of us aren't usually photographing this, though, so, the light, is, in fact, bouncing off of multicolored, multitextured, multitonal subjects throughout a scene. Modern cameras do a much better job of compensating for this, but still they're thinking of scenes as typically somewhere in the middle of the brightness scale. That means if you've got a scene at either extreme, really light or really dark, your meter's going to have problems.

A theoretical black cat in a black room, for instance, is definitely going to fool you camera's meter. How? Because your camera will effectively say, "Wow, not much light is bouncing off of this subject, I better give it a lot more exposure," and the next thing you know that dark subject in a dark scene is photographed as an overexposed, washed out gray that's a lot closer to the middle zone your camera thinks every scene consists of.

The same holds true for a white cat in a white room. Your camera, assuming all of those things should look gray, will underexpose and create a muddy gray mess instead of the high-key white-on-white scene you're in reality trying to photograph. If you've ever photographed a winter wonderland of a scene blanketed in white, you know exactly what I'm talking about. Instead of white, snow often looks gray. To fix it, you've got to outsmart your light meter.

So how do you beat your meter when it's trying to give you incorrect exposure information? Simple: you take control and adjust for the tonal values in a given scene at the extremes. And you do that by understanding the different tonal values (or zones, as Ansel Adams called them), using them to adjust the exposure compensation in auto exposure modes, or to make manual adjustments when shooting fully manual.

Digital Photo Tip Of The Week
There are 11 zones of tonal value that come from the world of black and white film photography, but I like to simplify things even further and start with only seven zones to correspond to seven full-stop differences in light. This is a great foundation on which to build even a colorful digital exposure. Thinking in grayscale and starting at pure black, the next tonal value would be black with detail (otherwise known as really dark gray), followed by dark gray, then middle gray, and now we're on to the lighter side of the dial. Next up is light gray, white with detail (or really light gray) and pure white, which is white without detail.

How does this help you beat your meter? Just determine into which "zone" the tones in a scene should fall and adjust the exposure compensation that number of stops. The black cat, for instance, should be rendered as black with detail, which is two stops from middle gray. You'll need to close down two stops from what your meter tells you if you'd like to make an accurate exposure. How you do that depends on the mode in which you're shooting.

In manual exposure mode, keeping an eye on the light meter you'll simply want to prepare yourself for a dark image of a black cat needing to register as about two stops underexposed. For a white cat, in a white hat, in a white room, you'd expect the meter to tell you the correctly exposed scene in fact appears practically two stops overexposed.

In auto exposure modes such as shutter priority, aperture priority or full program mode, you'll want to go into your camera's exposure compensation settings and adjust the camera to over- or underexpose accordingly. You can even do this compensation when working with a TTL flash metering. Since the black-on-black scene won't reflect as much light as the camera expects, it's not going to shut off the flash exposure until it's provided way too much illumination—two stops too much, in fact. So, instead of the camera's metering exposure compensation, adjust the flash exposure compensation instead. Understanding how the actual tones in a scene will impact the meter's performance will allow you to create more accurate exposures and better pictures in all sorts of atypically light or dark scenarios.

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